core[patch]: enable RunnableWithMessageHistory without config (#23775)

Feedback that `RunnableWithMessageHistory` is unwieldy compared to
ConversationChain and similar legacy abstractions is common.

Legacy chains using memory typically had no explicit notion of threads
or separate sessions. To use `RunnableWithMessageHistory`, users are
forced to introduce this concept into their code. This possibly felt
like unnecessary boilerplate.

Here we enable `RunnableWithMessageHistory` to run without a config if
the `get_session_history` callable has no arguments. This enables
minimal implementations like the following:
```python
from langchain_core.chat_history import InMemoryChatMessageHistory
from langchain_core.runnables.history import RunnableWithMessageHistory
from langchain_openai import ChatOpenAI

llm = ChatOpenAI(model="gpt-3.5-turbo-0125")
memory = InMemoryChatMessageHistory()
chain = RunnableWithMessageHistory(llm, lambda: memory)

chain.invoke("Hi I'm Bob")  # Hello Bob!
chain.invoke("What is my name?")  # Your name is Bob.
```

libs/core/langchain_core/runnables/history.py

@@ -537,8 +537,9 @@ class RunnableWithMessageHistory(RunnableBindingBase):
         configurable = config.get("configurable", {})
 
         missing_keys = set(expected_keys) - set(configurable.keys())
+        parameter_names = _get_parameter_names(self.get_session_history)
 
-        if missing_keys:
+        if missing_keys and parameter_names:
             example_input = {self.input_messages_key: "foo"}
             example_configurable = {
                 missing_key: "[your-value-here]" for missing_key in missing_keys
@@ -551,11 +552,16 @@ class RunnableWithMessageHistory(RunnableBindingBase):
                 f"e.g., chain.invoke({example_input}, {example_config})"
             )
 
-        parameter_names = _get_parameter_names(self.get_session_history)
-
         if len(expected_keys) == 1:
-            # If arity = 1, then invoke function by positional arguments
-            message_history = self.get_session_history(configurable[expected_keys[0]])
+            if parameter_names:
+                # If arity = 1, then invoke function by positional arguments
+                message_history = self.get_session_history(
+                    configurable[expected_keys[0]]
+                )
+            else:
+                if not config:
+                    config["configurable"] = {}
+                message_history = self.get_session_history()
         else:
             # otherwise verify that names of keys patch and invoke by named arguments
             if set(expected_keys) != set(parameter_names):

libs/core/tests/unit_tests/runnables/test_history.py

@@ -694,3 +694,24 @@ async def test_using_custom_config_specs_async() -> None:
             ]
         ),
     }
+
+
+def test_ignore_session_id() -> None:
+    """Test without config."""
+
+    def _fake_llm(input: List[BaseMessage]) -> List[BaseMessage]:
+        return [
+            AIMessage(
+                content="you said: "
+                + "\n".join(
+                    str(m.content) for m in input if isinstance(m, HumanMessage)
+                )
+            )
+        ]
+
+    runnable = RunnableLambda(_fake_llm)
+    history = InMemoryChatMessageHistory()
+    with_message_history = RunnableWithMessageHistory(runnable, lambda: history)  # type: ignore
+    _ = with_message_history.invoke("hello")
+    _ = with_message_history.invoke("hello again")
+    assert len(history.messages) == 4